Based upon technological advances in silicon based microelectronics, integration scales have been reached such that in a 1 cm2 silicon chip there may be billions of transistors and up to 10 km of metal connections. Signal propagation delays, because of parasitic resistors and capacitors (RC) among metal lines and intermetal dielectrics, poses the problem of achieving ever faster signal transmission with relatively small losses. A possible approach is to use optical signal transmission paths. In this way, the signal propagation speed is that of light in the optical medium, e.g. silicon oxide. Moreover a plurality of signals may be simultaneously transmitted through a single optical guide. Although silicon waveguides have a relatively large absorption due to free carriers that at 1.55 μm implies losses of several dB/cm for mono-modal waveguides, they are promising candidates for systems that could be electrically pumped, such as modulators, amplifiers, and the like.
In the field of integrated optics, light emitting devices have numerous applications for generating and conveying optical signals through a passive waveguide of an integrated device, and eventually for distributing the optical signal through splitters to several functional circuit blocks of a system-on-chip device.
Another device is the optical amplifier for compensating losses due to passive components (for example, splitters, and waveguides) and for pre-amplifying the signal for active components like detectors. As Erbium (Er) Doped Fiber Amplifiers (EDFA) are usually employed for communications over long distances, on the same working principle, integrated waveguide amplifiers, for example, have also been studied (Erbium-Doped Waveguide Amplifiers (EDWA)), and An EDFA may be optically pumped, and there is a vast literature on optically pumped EDWAs.
FIG. 1 is an exemplary hybrid system for pumping silicon optical amplifiers. The LED III-V is located inside a trench formed in the silicon substrate to optically pump the Erbium doped waveguide optical amplifier. In a host optical material, Erbium assumes the status of trivalent ion Er3+ with an electronic configuration [Xe]4f11 A schematic representation of the energy levels of Erbium is shown in FIG. 2. The characteristic wavelength of the transitions among levels are less sensitive to the host lattice (despite it causing the splitting of each degenerated level by the Stark effect) because the 4f shell is relatively strongly shielded by the other, more external, orbits may be completely occupied.
Optical amplification is based on the stimulated emission of an excited Erbium atom hit by a photon of an appropriate wavelength. Indeed, the transition 4I13/2→4I15/2 corresponds to the emission of a photon of λ=1.54 μm. As a result, an atom of Erbium in the excited state (4I13/2) decays to the main state (4I15/2) when hit by a photon of λ=1.54 μm, emitting a second identical photon. In this way, the initial signal is amplified.
Erbium excitation usually takes places by optical pumping: a photon of wavelength λ=0.98 μm, provided by a laser, is absorbed by an Erbium atom that, in this way, goes from the main state (4I15/2) to the second excited state (4I11/2), that has a very short mean life. From this state, the atom decays rapidly towards the more stable state 4I13/2, in which it remains for a relatively long time, sufficient to be hit by a photon at 1.54 μm (signal).
Nevertheless, the excitation cross-section of Erbium in a host matrix oxide is relatively low (10−21 cm2). Another optical system is that of forming Si nanocrystals in an oxide matrix and incorporating therein, Erbium ions. By doping oxide rich of silicon nanocrystals (silicon rich oxide or SRO), with Erbium a strong emission at 1.54 μm may be measured that is mediated by a process of direct energy transfer between the Si nanocrystal and the Erbium ion.
Silicon nanocrystals (Nc) efficiently transfer energy to the Er3+ ions by increasing the effective optical cross-section of Er up to 10−16 cm2. Moreover, solubility of Erbium is greater in silicon oxide than in crystalline silicon, and non radiating processes are suppressed because of the enlargement of the forbidden band in nanocrystals.
If a photon is absorbed by the Nc, an exciton is generated within the Mc itself. This exciton may recombine radiatively by emitting a photon with an energy that depends on the size of the Nc. If an Erbium ion is relatively close to the Nc, the exciton may non-radiatively recombine within the Nc and transfer its energy to the ion. A possible excitation model is depicted in FIG. 3.
Excitation takes place when two Erbium atoms that are at the level 4I13/2 combine their energies such that one of them decays to the main state (4I15/2) while the other is promoted to the state 4I9/2. Thus, because of this event, two atoms leave the level 4I13/2 without having given their contribution in terms of optical amplification.
An optically generated exciton (i), confined in the nanocrystal, gives up its energy (ii) to the Erbium ion. Then an electron of the Erbium atom is promoted to an unspecified excited state (iii) and decays towards the meta-stable state 4I13/2 see, for example, Polman and F. van Veggel, J. Opt Soc. Am B 21, 871 (2006), and A. J. Kenyon, C. E. Chryssou, C. W. Pitt, T. Shimizu-Iwayama, D. E. Hole, No. Sharma, C. J. Humphreys, Luminescence from erbium-doped silicon nanocrystals in silica: Excitation mechanisms, journal of applied physics volume 91, number 1, 1 January 2002.
Thus, either an emission (vi) of a photon at 1.54 μm or an upward conversion may occur, where by a further interaction with an excited nanocrystal, the excited atom passes from the metastable state 4I13/2 to a higher level (v) (See Kik, P. G., and Polman, A., J. Apl. Phys. (2002) 91 (1), 534). As it may be inferred from the scheme of FIG. 3, the system presents four levels: two related to the nanocrystal immersed in the oxide array and two related to the Erbium ion. As may be known, in such a system, inversion of population is more likely to be achieved. The literature reports attainment of a net gain in a waveguide amplifier made of SRO+Er (EDWA) by optical pumping; See for example, Han, H.-S., et al., Appl. Phys. Lett. (2002) 81 (20), 3720 and Kik, P. G., and Polman, A., J. Appl. Phys. (2002) 91 (1), 534.
Usually, in waveguide optical amplifiers, light is guided in a core material of higher refraction index than adjacent media, for example, a silicon core and a silicon oxide cladding or mantel. Recently, a type of waveguide, called “slot waveguide,” has been developed by introducing a thin layer of a relatively low refraction index material, typically SRO+Er, between two silicon layers of high refraction index. A characteristic of this type of waveguide is to focus the electric field in the region of lower refraction index. FIG. 4 shows the working scheme of a vertical “slot waveguide” (a) and the transversal electric optical mode that is confined in the slot (b) (See, for example, C. Barrios and M. Lipson, Optics Express 13 (25), 10092 (2007)).
The physical effect that allows guiding of light inside a waveguide is the total internal reflection that allows the creation of particular states, the “guided modes”. Thus a guided mode is a state allowed inside the considered guiding structure. To calculate the guided modes, the field is assumed to be a planar transversal monochromatic electromagnetic waveform, and appropriate boundary conditions are imposed.
Also, the distribution of the electromagnetic field is conditioned by geometric, structural and physical characteristics of the waveguide. The fields, internal and external to the waveguide, may be found by solving Maxwell's equations. Outside the thin layer of the low refraction index of the guide, the electromagnetic field is evanescent. The extinction coefficient of the mode has a smaller value for the modes that propagate in materials of higher refraction index. The largest the refraction index, the more the evanescent mode propagates outside.
In a “slot waveguide”, the two layers of high refraction index, compared to the lower refraction index of the thin silicon core layer, are close enough to each other that the evanescent fields that propagate in the two layers of high refraction index add up together, which enhances the confinement of the electromagnetic field. In fact, such an effect of enhanced confinement is typically not observed in “slot” waveguides for silicon core thicknesses exceeding the reciprocal of the extinction coefficient of the guiding silicon oxide layers. In horizontal confinement structures, as represented in FIG. 5, it is the transversal magneto-optic mode that is confined. For the specific function of a waveguide, the advantage of concentrating the electromagnetic field in relatively very thin layers having a relatively low refraction index is to reduce light transmission losses because of absorption along the waveguide.